This invention relates to oscillator circuits, and more particularly, to an I.sup.2 L ring oscillator circuit with adjustable repetition rate.
Ring oscillator circuits are desirable because they may be fabricated as an integrated circuit, ordinarily as part of an integrated system, and do not require any external components. The ring oscillators are sufficiently accurate to provide digital clocking signals. However, the frequencies of ring oscillators of this type are not accurately predictable and variations in processing can greatly alter the repetition rate of the oscillator in any given system. Furthermore, since the entire oscillator is integrated, there is no means of adjusting the oscillator once fabricated. In certain instances, such as the use of the ring oscillator for a dual slope analog-to-digital converter or other system in which frequency accuracy and stability are important, prior art has been unable to utilize ring oscillators; instead, the more expensive adjustable oscillators with external components, such as variable resistors or capacitors, have been required.
It is, therefore, an object of the present invention to provide an improved I.sup.2 L ring oscillator circuit.
Another object of the invention is to provide an accurate but inexpensive ring oscillator circuit which is capable of being integrated on a single semiconductor substrate with other I.sup.2 L circuitry.
A further object of the invention is to provide an I.sup.2 L ring oscillator circuit which includes means by which the frequency may be adjusted during or after fabrication is complete.
It is another object of the invention to provide a method of fabricating an I.sup.2 L system including an I.sup.2 L ring oscillator circuit which provides for the adjustment of the ring oscillator circuit to compensate for unpredictable variations in parameters caused by processing.
It is a further object of the invention to provide an I.sup.2 L ring oscillator circuit for a dual slope analog-to-digital converter, both being integratable on a single substrate utilizing I.sup.2 L techniques.
Yet another object of the invention is to provide an I.sup.2 L ring oscillator circuit for a portable, battery-operable, digital thermometer.